Commit 8ab57aa added a yield to the buffer-copy loop, which means that
the copy can yield before done and the multishard reader might see the
half-copied buffer and consider the reader done (because
`_end_of_stream` is already set) resulting in the dropping the remaining
part of the buffer and in an invalid stream if the last copied fragment
wasn't a partition-end.
Fixes: #11561
(cherry picked from commit 0c450c9d4c)
86dbbf12cc